As the focal depth of optical steppers decreases, it becomes more important to determine the position of best focus accurately and quickly. Semiconductor CMOS chips are manufactured on silicon wafers by a sequence of material additions (i.e., low pressure chemical vapor depositions, sputtering operations, etc.), material removals (i.e., wet etches, reactive ion etches, etc.) and material modifications (i.e., oxidations, ion implants, etc.). In order to build very small, electrically active devices on the wafer, the impact of these operations has to be confined to very small, well-defined regions on the wafer.
Lithography, in the context of manufacturing CMOS devices, is the process of patterning openings in photosensitive polymers. These openings define small areas in which the silicon base materials are modified by a specific operation in a sequence of processing steps. The manufacturing of CMOS chips involves repetitive patterning of photoresist, followed by an etch, implant or deposition, and ending with the removal of the expended photoresist to make way for the new resist to be applied for other iterations of this process sequence.
The basic lithography system consists of a light source, a stencil or photomask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask. Since a wafer containing from fifty to one hundred chips is patterned in steps of one to four chips at a time, these lithography tools are commonly referred to as steppers.
The resolution R of an optical projection system, such as a lithography stepper, is limited by the parameters described in Rayleigh's equation: EQU R=K*(.lambda./NA) (1)
where .lambda. is the wavelength of the light source used in the projection system and NA is the numerical aperture of the projection optics used. K is a factor describing how well a combined lithography system can utilize the theoretical resolution limit in practice, and can range from 0.5 to 0.8 for standard exposure systems.
Conventional photomasks, commonly referred to as chrome on glass (COG), consist of chromium patterns on a quartz plate, allowing light to pass wherever the chromium has been removed from the mask. Light of a specific wavelength is projected through the mask onto the wafer, exposing the resist wherever hole patterns are placed on the mask. Exposing the resist to light of an appropriate wavelength causes modifications in the molecular structure of the resist polymers which, in turn, allow a developer to dissolve and remove the resist in the exposed areas. The photomask, when illuminated, can be pictured as an array of individual, infinitely small light sources that can be either turned on or turned off. If the amplitude of the electric field vector, which describes the light radiated by the individual light sources, is mapped across a cross section of the mask, a step function may be plotted reflecting the two possible states that each point on the mask may have.
A perfectly square step function exists only in the theoretical limit of the exact mask plane. At any distance away from the mask, such as in the wafer plane, the diffraction effect will cause images to exhibit a finite image slope. At small dimensions, that is, when the size of the images to be printed are small relative to .lambda./NA, electric field vectors of adjacent images will interact and add constructively. The resulting light intensity curve between the features is not completely dark, but exhibits significant amounts of light intensity created by the interaction of adjacent features. The resolution of an exposure system is limited by the contrast of the projected image, that is, the intensity difference between adjacent light and dark features. An increase in the light intensity in nominally dark areas will eventually cause adjacent features to print as one combined structure rather than discrete images. The quality with which small images can be replicated in lithography depends largely on the available process latitude, that is the amount of allowable dose and focus variation that still results in correct image size.
Determination of optical focus in photolithography has always been a time consuming and relatively uncertain process. Traditionally, focus is determined by exposing a matrix field through a range of focus settings, then inspecting the resultant pattern for the best-focused images. Operators may be quite good at this, however, the process is slow and inherently subjective. A variety of automated methods for determining focus have been developed but most of these methods use an aerial image monitor to determine the position of best focus. The wafer is then positioned at the best focus location by mechanical means. These methods, however, are susceptible to slight drifts in the positioning mechanism or changes in the required focus offset induced by changes in the film stack from one batch of wafers to the next.
Best focus can also be determined by phase shifted lithography where the destructive interference caused by a 180.degree. phase transition across a critical dimension feature is used to improve the lithographic process window. Phase shifted lithography improves the lithographic process latitude or allows operation of a lower K value by introducing a third parameter on the mask. The electric field vector has a magnitude and direction, so in addition to turning the electric field on and off, it can be turned on with a 0.degree. phase or turned on with a 180.degree. phase. This phase variation is achieved by modifying the length that a light beam travels through the mask material. By recessing the mask an appropriate depth, light traversing the thinner portion of the mask and light traversing a thicker portion of the mask will be 180.degree. out of phase. One problem with phase shifted mask design are the narrow lines printed by the destructive interference of light across residual phase edges, such as, phase edges that are present due to the topology of the layout design but that do not contribute constructively to the patterning process.
One conventional approach, which does not require a second exposure, is the conjugate twin shifter approach. This is described in, Conjugate Twin-Shifter for the New Phase Shift Method to High Resolution Lithography, Proc. SPIE, pp. 112-123, Vol. 1463, 1991. In this approach, phase shifting is accomplished not by a transition from 0.degree. to 180.degree. across the critical dimension feature, but rather by a 90.degree. to 270.degree. transition. This results in a 180.degree. phase difference across the desired feature and only a 90.degree. phase transition with respect to the background. The result is that the residual phase edge images with reduced contrast and exposure conditions can be adjusted so as not to print these unwanted patterns.
A phased focus monitor is disclosed in U.S. Pat. No. 5,300,786 issued to Brunner et al. The monitoring system in Brunner et al. includes a focus test mask disposed between the illumination part and the lens that projects a test pattern on the object surface. This focus monitor has drawbacks, however, in that an overlay tool is required in order to read the focus.